Fuel cells are devices that release electrical energy using an electrochemical reaction. A major class of fuel cells utilizes hydrogen fuel, and the electrochemical oxidation of hydrogen to water is catalyzed using electrodes containing precious metal catalysts. Precious metal catalytic elements for use in precious metal catalysts include, but are not limited to, platinum (Pt), ruthenium (Ru), palladium (Pd), gold (Au), and rhodium (Rh). It is widely accepted that the high cost and limited supply of platinum and other catalytic elements are obstacles to large scale commercialization of fuel cells.
There are several types of fuel cells. Most common is the polymer electrolyte membrane (PEM) fuel cell. The PEM forms the basis for a membrane electrode assembly (MEA), which is the structure by which hydrogen can be oxidized to generate electricity. An anode (i.e., a negative electrode) is provided on one side of the PEM and a cathode (i.e., a positive electrode) is provided on the opposite side of the PEM. The anode contains a catalyst, typically comprising platinum, for promoting dissociation of hydrogen into electrons and positive hydrogen ions. The cathode also contains a catalyst, typically comprising platinum, for promoting reduction of oxygen. An MEA typically carries a catalytic element loading between about 0.5 mg/cm2 and 4 mg/cm2, although recent research has obtained effective performance with catalytic element loadings as low as 0.15 mg/cm2. Typically, these loadings in current commercial fuel cells translate to about 0.5% to 3.0% by weight of catalytic element in the MEA.
A commonly used polymer electrode membrane is Nafion™ by E.I. DuPont de Nemours Company. Nafion™, a Teflon™-based polymer, is a sulfonated perfluropolymer. Even when using a membrane that is itself free of fluorine, a perfluropolymer ionomer is typically mixed into the electrocatalyst layers (i.e., the anion and the cation) to improve the mobility of the positive hydrogen ions. Additionally, the presence of a fluoride-rich polymer makes the powder of the MEA hydrophobic when the MEA is ground.
In one type of fuel cell, the anode and cathode are coated onto the PEM to form a catalyst coated membrane (CCM). A CCM fuel cell can include platinum, ruthenium, and other catalytic elements. In another type of fuel cell, a carbonaceous gas diffusion layer is applied to the anode and another carbonaceous gas diffusion layer is applied to the cathode to form gas diffusion electrodes (GDEs). A GDE fuel cell can also include platinum, ruthenium, and other catalytic elements. The gas diffusion layers provide for the uniform distribution of hydrogen and oxygen to their respective sides of the PEM, provide a conductive pathway for electricity to be transmitted out of the fuel cell, and provide a porous means for the water produced by the electrochemical reaction to be transported away.
Another type of fuel cell using catalytic elements such as platinum is a alkaline fuel cell (AFC). Still another type of fuel cell using catalysts is a phosphoric acid fuel cells (PAFC), which can use a polybenzylimidazole (PBI) membrane or a silicon carbide matrix saturated with phosphoric acid electrolyte. Regardless of the type, after a period of use, a fuel cell often must be replaced, due to fouling of the catalyst, or for another reason. In particular, after repeated cycling of the fuel cell during operation (i.e., cycling between periods of high and low voltage generation), the catalyst can be become less effective. Rather than simply disposing of a fuel cell that must be replaced, it is highly desirable to recover as much catalytic element as possible from the MEA, due to the value of the precious metal catalytic element.
The conventional approach to recover platinum and other precious metal catalytic elements from an MEA includes combusting the PEM and the carbonaceous diffusion layers, dissolving the resultant ash in acid, and purifying the precious metal using standard refining chemistry. However, the high fluorine content of the MEA, particularly those with Nafion™ or other Teflon™-based membranes, results in toxic emissions of hydrogen fluoride gas (HF) and other fluorine compounds from the combustion process. Therefore, a high degree of catalytic element recovery, without the need to combust the MEA, is desired for a recovery method to be commercially viable.